1. Field of the Invention
The present invention relates to an image recording apparatus, an image processing apparatus, an image processing method and a computer-readable medium, and more particularly, to an image recording apparatus, an image processing apparatus, an image processing method and a computer-readable medium which correct density non-uniformities caused by variations in the characteristics of respective recording elements when recording an image on a recording medium (recording paper) using a recording head in which a plurality of recording elements are provided.
2. Description of the Related Art
In the inkjet recording apparatus, from the viewpoint of stabilizing print quality, it is important to make the ink ejection volume uniform from each of a plurality of nozzles which are formed in a recording head. In order to make the ink ejection volume from each nozzle uniform, the ink ejection volume from each nozzle when a prescribed value is input is measured, and the input values for the respective nozzles are corrected on the basis of the measured ejection volume. This measurement of the ejection volume is carried out, for example, by outputting a test chart and reading in the test chart by means of a scanner.
Japanese Patent Application Publication No. 2005-141232 discloses technology which converts position information in a scanner image into position information in image data, on the basis of position information obtained from a reference mark included in a test chart, when correcting density non-uniformities in the output of images on the basis of density information acquired from a test chart for density measurement. According to this technology, it is possible to correct density non-uniformities appropriately, even if there is divergence between the pixels of the scanner used to scan the test chart and the pixels in the image forming device that is used to print the test chart.
Here, a brief outline of Japanese Patent Application Publication No. 2005-141232 will be described. FIG. 15 is a diagram illustrating a scan image read in by scanning a test chart output in a case where the nozzle ejection characteristics are normal, and image data converted to the nozzle resolution on the basis of this scan image.
As illustrated in FIG. 15, the test chart is constituted by a gray level portion and reference marks. The gray level portion is printed by inputting the same density value to all of the nozzles. Furthermore, the reference marks are printed at prescribed intervals in the breadthways direction of the gray level portion, and serve as guide marks when allocating the density data in the breadthways direction of the gray level portion with the respective nozzles.
The central position of the reference marks is extracted from the scan image of the test chart, and a gray level profile is created by setting the density of the gray level portion as a function of the position in the breadthways direction. Moreover, the gray level profile is converted from scanner pixel units to nozzle pixel units (image data pixel units) on the basis of the central position of the reference marks thus extracted, and the local TRC (tone reproduction curve: a density conversion curve for each pixel) is calculated with respect to the image data pixels.
In this case, the left-end scan pixel position Xss and the right-end scan pixel position Xse when facing the image of reference marks are judged to be the lateral direction boundaries of the gray level portion, and are converted respectively into an image data pixel position Xds and an image data pixel position Xde. In other words, the gray level portion is converted in image data pixel units to an image from Xds to Xde.
Here, a case where there is an ejection failure nozzle (non-ejection nozzle) in the nozzles which print the reference marks will be described. FIG. 16 is a diagram illustrating a scan image read by scanning a test chart output when there is an ejection failure nozzle, and image data for density correction generated on the basis of this scan image, and depicts a case where the left-end nozzle when facing the image is suffering ejection failure.
As illustrated in FIG. 16, since the left-end nozzle is suffering ejection failure, then of the reference marks which ought to be printed, the left-end reference mark is not printed. Therefore, the scan pixel position Xss′ printed at the furthest left-hand end of the reference marks of the scan image is judged to be the left-side boundary of the gray level portion and is converted into an image data pixel position Xds. More specifically, the gray level portion is converted in image data pixel units, to an image from Xds to Xde, but since Xds is allocated to a position different to the intended position Xss, then the scan pixel positions are not allocated correctly to the image data pixel positions.
In this way, according to the technology in Japanese Patent Application Publication No. 2005-141232, if there is an ejection failure nozzle in the nozzles which print the reference marks, the pixels of the scan image and the image data are not allocated correctly, and therefore density non-uniformities cannot be corrected appropriately. Japanese Patent Application Publication No. 2005-141232 does not disclose ways of avoiding this problem.